In optical communication, a 1550 nm wavelength band is commonly used because of the minimum loss in optical fiber. In order to put a next-generation quantum communication into practical use, it is essential to develop a high speed single-photon detector in this particular communication band. An InGaAs/InP APD is currently used for detecting a single-photon in the 1550 nm wavelength band because it is most sensitive in such wavelength (see non-patent documents 1, 2 and 3 as listed hereunder). A method of detecting a single photon using an APD is basically the Geiger mode method. There are two methods as improvements of the Geiger mode method: one is a gating method and the other is an active-quenching method.
After pulsing is the most influential factor of limiting the repetition rate in photon detection using an APD. After pulsing is a phenomenon in which fractional parts of avalanche electrons are trapped in a defect level or the like and subsequently released at a certain time, thereby causing a new avalanche and outputing a pulse. Probability of causing such after pulsing is proportional to the intensity of the avalanche current that flows in the APD and decreases with time after termination of avalanche multiplication. After pulsing can be suppressed by decreasing the bias voltage and thus the magnitude of avalanche. Unfortunately, however, it becomes more difficult to detect photons when the magnitude of avalanche is small.
In the Geiger mode, a DC bias voltage higher than the breakdown voltage is applied to the APD and the resulting avalanche multiplication current caused by photons is converted into voltage for detection of the photons. When operating an InGaAs/InP APD in the Geiger mode, the probability of after pulsing is very large. As a result, it is impossible to detect subsequent photons until termination of the avalanche current and after pulsing noise, thereby enabling to detect only about one photon in every one microsecond. This is the reason why the APD is operated in the gated mode so that the APD is in the Geiger mode for only a short time.
In the gated mode, the APD is kept in the Geiger mode for about only 1 ns at the expected photon arrival time in order to reduce after pulsing noise. As shown in FIG. 18, a DC voltage lower than the APD breakdown voltage superimposed with a gate pulse signal is applied so that the bias voltage exceeds the breakdown voltage of the APD for a short time of about 1 nanosecond. The avalanche multiplication current caused by an incident photon is converted into voltage for detecting the photon. In case of using a rectangular-wave as the gate signal for photon detection, a detection threshold voltage must be set higher than the gate signal component (charge pulse) voltage in the output signal. As a result, in case of phone detection, the avalanche multiplication current must be in excess of the threshold voltage. A quantum efficiency, that represents the rate of causing avalanche by each photon, is proportional to the voltage applied to the APD. A dark count probability, that represents the probability of causing avalanche by thermal noise, is also proportional to the voltage applied to the APD. Therefore, the quantum efficiency and the dark count probability are always in the trade-off relationship. In a typical operational example, operation temperature is −30 degrees centigrade (243K), repetition frequency is about 100 kHz and the gate signal is several nanoseconds in width (see the non-patent documents 3 and 4).
In an active-quenching method, after pulsing noise is suppressed by forcedly stopping avalanche multiplication upon photon detection. As soon as recognizing occurrence of avalanche multiplication due to photon detection while applying to the APD a voltage in excess of its breakdown voltage, the APD voltage is decreased less than the breakdown voltage for terminating the avalanche multiplication. Thereafter, the voltage in excess of the breakdown voltage is applied to the APD again. Photons are detected continuously by repeating the foregoing operation. Some conventional examples relating to photon detection will be described hereunder.
A patent document 1 discloses a highly efficient single photon detector that is sensitive in the neighborhood of 1550 nm wavelength. An InGaAs-APD that is sensitive in the neighborhood of 1550 nm wavelength is operated in the Geiger mode. The APD is cooled during the operation in the gated mode by GPQC.
A patent document 2 discloses a single photon detection method for accurately detecting incidence of a single photon. Generated is a predetermined voltage lower than the APD breakdown voltage. A pulse voltage that is in excess of the breakdown voltage when combined with the predetermined voltage is generated in synchronism with a trigger signal that is generated simultaneously with outputting a photon from the single photon generation source. The photon is permitted to enter the APD when the predetermined voltage and the pulse voltage are applied to the APD, thereby detecting incidence of the photon when the output level of a pulse signal from the APD exceed the predetermined threshold level. The timing of applying the pulse voltage to the APD is adjusted to the optimum timing that is close to the incidence of the photon.
A patent document 3 discloses a single photon detector capable of changing the setting of a reference value corresponding to the quiescent time and easily removing a produced after pulse. A photon is detected by single photon detection means that comprises an APD as a light receiving device. There is control means for storing all detection times. It is determined to be a valid detection only if the difference between the current and the next previous photon detection times is longer than the preset reference value, thereby eliminating the after pulse.
A patent document 4 discloses a long distance quantum encryption system for extending the transmission distance by reducing photon loss in a receiver section and using a single photon detector of high efficiency, low dark count probability. A laser beam pulse of 1550 nm in wavelength is split into a horizontally polarized reference optical pulse and a vertically polarized signal optical pulse in an encryption key information receiver section. The signal optical pulse is delayed and transmitted. In an encryption key information transmitting section, the polarization surface of the reference optical pulse is rotated by 90 degrees. A random-phase shift is provided to the signal optical pulse to rotate the polarization surface by 90 degrees. These pulses are attenuated to be formed into single-photon pulses and are sent back. In the encryption key information receiving section, a random phase shift is provided to a feedback reference optical pulse for delaying it and superimposed with a feedback signal optical pulse. It is then split by a polarization optical splitter in response to its polarization state and is detected by an APD that is in a quiescent state only for a release time of trapped carriers, thereby obtaining the encryption key information.
A patent document 5 discloses a photon detector for detecting photon without removing noise due to charging pulses of an APD. A DC voltage lower than the breakdown voltage is applied to an APD as its bias voltage. A narrow voltage pulse is superimposed with the bias voltage at the forecasted photon arriving time. Since noise due to the charging pulses decreases when photon arrives to cause an avalanche current, this phenomenon is detected by a pulse height descriminator for judging photon arrival.
Reported in the non-patent document 1 is a 1550 nm single photon detector using a thermoelectrically cooled InGaAs-APD. It is a gated mode single photon detector sensitive to 1550 nm light using a thermoelectrically cooled InGaAs-APD. At the operation temperature of 238K, it exhibited a quantum efficiency of 24.3% with a dark count probability per gate of 9.4×10−5.
The non-patent document 2 reports a photon counter using a Peltier cooled InGaAs-APD for quantum key distribution. The photon counting performance of three types of electronically cooled InGaAs/InP-APDs is studied at 1550 nm. The best result showed a dark count probability of 2.8×10−5 per gate (2.4 ns) at the detection efficiency of 10% and temperature of −60 degrees centigrade. The after pulsing probability and the timing jitter are also studied for comparison to other reports and application to simulation in the quantum key distribution system. In a 54 km transmission, the achievable error rate appears to be 10%.
The non-patent document 3 reports a single photon detector for long-distance fiber optic quantum key distribution. The photon counting performance of a liquid nitrogen cooled InGaAs/InP-APD is studied at 1550 nm. The quantum efficiency of 13.7% was obtained at −55 degrees centigrade, while the dark count probability per gate (1 ns) was kept as small as 2.4×10−5. The single photon detector could achieve a 104.4 km fiber-optic quantum key distribution under the ideal condition.
The non-patent document 4 reports an avalanche photodiode and a quenching circuit for single photon detection. The APD is connected to an avalanche quenching circuit in the Geiger mode and operates above the breakdown voltage for detecting single photon. It is therefore known as a single photon avalanche diode SPAD. Circuit configurations suitable for this operation mode are studied in greater detail for assessment of its relative merits in photon counting and operation timing. Although a simple passive quenching circuit (PQC) is useful for SPAD device testing and selection, it has fairly limited applications. A properly designed active quenching circuit (AQC) is able to exploit the best performance of SPAD. A thick film silicon SPAD that operates at a high voltage (such as 250V-450V) exhibits the photon detection efficiency higher than 50% in the wavelength range of 540 nm-850 nm wavelength and as high as 3% at 1064 nm. A thin film silicon SPAD that operates at a low voltage (such as 10V-50V) exhibits the detection efficiency of 45% at 500 nm but decreases to 10% at 830 nm and to as low as 0.1% at 1064 nm. The time resolution achieved in photon timing is 20 ps (FWHM) with a thin film silicon SPAD, while it ranges from 350 to 150 ps (FWHM) with a thick film silicon SPAD. The achievable minimum counting dead time and maximum counting rate are 40 ns and 10 Mcps with a thick film silicon SPAD, while 10 ns and 40 Mcps with a thin film silicon SPAD. A germanium and III-V compound semiconductor SPAD may extend the photon counting range to at least 1600 nm or the near-infrared region.
The non-patent document 5 reports a high performance unitary single photon detector for communication wavelengths. A commercially available APD and a circuitry necessary for operating it as a single photon detector (SPD) are assembled on a single PC board (PCB) as a unit. At temperatures achievable with a Peltier cooler (e.g., 200K to 240K), the unitary single photon detector achieves high detection efficiency (DE) at 1308 nm and 1545 nm with low dark count probability (e.g. about 10−6/bias pulse at DE=20%, 220K), thereby making it useful for a quantum key distribution (QKD). High speed bias pulses are generated for canceling transient noise and amplifying the signal for sending it to a unitary discriminator. A digital blanking circuit suppresses after pulsing.
The non-patent document 6 reports a balanced gated mode photon detector for quantum bit discrimination at 1550 nm. A photon detector is developed by combining a pair of APDs for quantum bit discrimination at 1550 nm. Spikes accompanied with the signal in the gated mode are canceled by the balanced outputs from the two APDs. The spike cancellation enables one to reduce the threshold in the discriminators and thus the gate pulse voltage. The dark count probability and after pulsing probability were reduced to 7×10−7 and 7×10−4, respectively, without affecting the detection efficiency (11%) at 178K.
The non-patent document 7 reports a photon counter at 14 MHz using a room temperature InGaAs/InP-APD. Developed is a high speed gated mode single photon counter using an InGaAs/InP-APD at 1550 nm wavelength. It operates at room temperature up to 14 MHz with after pulsing probability at or below 0.2%. It exhibits the optimum equivalent noise power of 2.2×10−15 WHz−1/2 at 14% quantum efficiency with dark count probability of 0.2%. An equivalent noise power per gated frequency is used as a barometer for comparing high speed photon detectors. With this barometer, this system outperforms previously reported counters at 1550 nm wavelength. It demonstrates that for gate width of a nanosecond or below, amplitude difference distributions of dark versus light counts allow an optimum decision threshold to be set for a given bias voltage.
The non-patent document 8 reports a 10.5 km optical fiber quantum key distribution at 1550 nm wavelength with a key distribution rate of 45 kHz. A high-speed single-photon detector is crucial for efficiently performing quantum key distribution. A key distribution over an optical fiber is performed by using light of 1550 nm wavelength, and a single photon detector operating at 10 MHz. In order to supress after pulse generation, a single photon is detected by the discharge pulse counting. Furthermore, by setting a dead time in the single photon detection, after pulsing is neglected and the number of bit errors in the quantum key distribution is reduced. In the quantum key distribution, achieved is the key distribution rate of 45 kHz with the bit error rate of 2%. The length of the optical fiber is 10.5 km.
Patent document 1: JP2003-142724A
Patent document 2: JP2003-243694A
Patent document 3: JP2003-282933A
Patent document 4: JP2003-289298A
Patent document 5: JP2005-114712A
Non-patent document 1: A. Yoshizawa, and H. Tsuchida, “A single-photon detector using a thermoelectrically cooled InGaAs avalanche photodiode”, Jpn. J. Appl. Phys., vol. 40, no. 1, pp. 200-201, 2001
Non-patent document 2: D. Stucki, G. Ribordy, A. Stefanov, H. Zbinden, J. G. Rarity, and T. Wall, “Photon counting for quantum key distribution with Peltier coold InGaAs/InP APD's”, J. Mod. Opt., vol. 48, no. 13, pp. 1967-1982, 2001.
Non-patent document 3: N. Namekata, Y. Makino, and S. Inoue, “Single-photon detector for long-distance fiber optic quantum key distribution”, Opt. Lett., vol. 27, no. 11, pp. 954-956, 2002.
Non-patent document 4: S. Cova, M. Ghioni, A. Lacaita, C. Samori, and F. Zappa, “Avalanche photodiodes and quenching circuits for single-photon detection”, Appl. Opt., vol. 35 no. 12, pp. 1956-1976, 1996.
Non-patent document 5: D. S. Bethune, W. P. Risk, and G. W. Pabest, “A high-performance integrated single-photon detector for telecom wavelengths”, J. Mod. Opt., vol. 51, no. 9-10, pp. 1359-1368, 2004.
Non-patent document 6: A. Tomita and K. Nakamura, “Balanced gated-mode photon detector for quantum-bit discrimination at 1550 nm” Opt. Lett., vol. 27, no. 20, pp. 1827-1829, 2002.
Non-patent document 7: P. L. Voss, K. G. Koprulu, S.-K. Choi, S. Dugan, and P. Kumar, “14 MHz rate photon counting with room temperature InGaAs/InP avalanche photodiodes”, J. Mod. Opt., vol. 51, no. 9-10, pp. 1369-1379, 2004.
Non-patent document 8: A. Yoshizawa, R. Kaji, and H. Tsuchida, “10.5 km fiber-optic quantum key distribution at 1550 nm with a key rate of 45 kHz”, Jpn. J. Appl. Phys., vol. 43, no. 6A, pp. L735-L737.